1. Field of the Invention
This invention relates to sliding contact-making structures constituted by a cylinder and a piston of an internal combustion engine installed in an automobile, motorbike and the like, in particular, to such sliding contact-making structures allowing for abating friction loss between the inside surface of the cylinder and the sliding surface of the piston rings with the use of a decreased amount of lubricating oil. This invention also relates to an internal combustion engine comprising the above sliding contact-making structures, which has a compact and lightweight piston.
2. Background of the Art
In a combustion engine, a piston is fitted into a cylinder, and reciprocally slides upon the inside surface of the cylinder at a high speed at a high temperature when operated. As a result, frictional wear or seize tends to occur between the inside surface of the cylinder and the sliding surface of the piston. In order to prevent such a problem, a lubrication oil is normally supplied to the surfaces in sliding contact.
Further, it is hitherto known that the inside surface of the cylinder is finished with a honing (striation) treatment so as to form myriad grooves thereon to retain the lubricating oil externally supplied therein, whereby the effects of the lubricating oil on the surfaces in sliding contact is enhanced.
However, the following drawbacks are associated with the above structures:
First, the grooves must have a depth, e.g., a surface roughness of approximately 3 .mu.mRz or more ("Rz" is defined in ISO 4287 or JIS B0601) sufficient for exercising lubricating effects of lubricating oil kept therein; otherwise, frictional wear or seize will occur due to insufficient retention of lubricating oil. As a result, the surface roughness of the inside surface is increased, thereby increasing coefficient of friction on the surfaces in sliding contact, notwithstanding the lubricating oil is used. Accordingly, due to friction loss occurring on the surfaces in sliding contact, engine performance suffers.
Secondly, consumption of lubricating oil which is supplied to the surfaces in sliding contact is inherently increased since the grooves formed by a honing treatment are deep to retain a sufficient amount of lubricating oil therein for abating the friction coefficient.
Thirdly, decrease in weight and size of a piston cannot be achieved since, especially in a cylinder with a high surface roughness, the sliding area of the piston must be enlarged by sufficiently extending the skirt area of the piston in the axial direction in order to reduce the pressure per area exerted on the piston; otherwise, frictional wear of the surfaces in sliding contact tends to be locally intensified due to high surface roughness of the inside surface of the cylinder. A long skirt area of the piston brings about increase in weight or size of the piston.
Fourth, in the case of a cylinder having a sleeve inserted inside, uneven pressure between the inside surface of the cylinder and the piston inherently occurs because the inside surface of the cylinder is prone to heat deformation due to low thermal conductivity of the cylinder (low diffusion of heat) as well as a difference in thermal expansion coefficients between the cylinder block (aluminum alloy) and the sleeve (ion cast) inserted inside the cylinder block. As a result, the sliding surface of the piston mainly touches a projected part of the inside surface of the cylinder. If the inside surface is coarse due to deep grooves, the surfaces in sliding contact will be locally and intensively worn out. Although a long skirt area of the piston mitigates the above problem by reducing the pressure per sliding area, it brings about increase in weight or size of the piston, which affects engine performance.